Miniaturized atomic clocks characterized by a small size and a drastically reduced power consumption compared to standard atomic clocks exhibit an increasing interest mainly for applications in portable devices. The unprecedented frequency stability of atomic clocks is achieved by a suitable interrogation of optically excited atoms in order to achieve the hyperfine splitting of the electron state of the reactant, which takes place in the so-called vapor cell, the heart of an atomic clock. The vapor cell comprises a sealed cavity, which contains small amounts of suitable reactants: an alkali metal, preferably rubidium or cesium, buffer gas(es), and/or anti-relaxation coating(s).
MEMS technology allows for fabricating miniaturized vapor cells having a volume in the range of a few cubic millimeters. Silicon micromachining is particularly interesting. It allows a very high level of miniaturization and hybrid integration with control electronics and sensors, and the wafer-level batch fabrication affords a low cost production and higher reproducibility.
Various atom excitation techniques have been investigated concerning the field of miniaturized atomic clocks developments. One alternative includes coherent population trapping (CPT) by means of a modulated laser, while another alternative is based on double-resonance (DR) microwave excitation by means of a modulated magnetic field.
In most vapor cell frequency references, which do not use CPT or DR, the minimum size of the clock physics package is determined in part by the cavity that confines the microwaves used to excite the atoms. This cavity is usually larger than one-half the wavelength of the microwave radiation used to excite the atomic resonance. For cesium and rubidium, this wavelength is of the order of several centimeters, clearly posing a problem for the development of vapor cell references for portable applications. Thus, CPT or DR excitation is very suitable for micro-machined vapor cells.
Indeed, L-A Liew, Appl. Phys. Lett. 84, 2694 (2004) discloses a method to fabricate millimeter sized cesium vapor cells using silicon micromachining and anodic bonding techniques, where the frequency reference is based on optical excitation and CPT interrogation. The results presented in this work show that it is possible to design and build frequency references far smaller than known in the prior art before even if it results in a complicated interrogation optics assembly highlighted by the miniaturization conditions.
In addition, in order to realize a working CPT or DR atomic clock a magnetic field has to be provided which is required to be homogeneous inside the vapor cell in order to achieve ground-state hyperfine splitting of the alkali atoms.
There are different ways to create the needed homogeneous magnetic field.
One option is the use of permanent magnets, but they present the disadvantage that the strength of the magnetic field cannot be adjusted, and that they make the final device quite bulky.
On the other hand, electromagnets could be used for achieving a proper homogeneous magnetic field. Helmholtz configuration with two planar coils integrated directly on the two windows of the vapor cell may be a suitable option. However, the Helmholtz condition r=d (where r is the radius of the coil and d is the distance between the two coils) must be fulfilled in order to obtain a homogeneous magnetic field, a requirement which limits downsizing of the vapor cells. Moreover, planar coils realized in MEMS technology are characterized by a very low thickness of the coil, typically in the range of some hundreds of nanometer. As a consequence, a planar coil has a relatively high electrical resistance and hence an elevated power dissipation. Thus, a skilled person is not encouraged to investigate planar coils for providing a homogeneous magnetic field in a miniaturized vapor cell.
The object of this invention is to at least partially overcome the limitations described, and thereby provide a versatile simple configuration using electromagnets to create the needed homogeneous magnetic field on the vapor cell boosting the methods of miniaturization and providing a favorable simplicity to efficiency ratio.